There are many needs that can be satisfied by a lock which permits easy interchange of keys. This may be necessary for security where money is involved as in vending machines. It may be necessary for households after a burglarly or a change of occupancy. Change in hotel rooms usually require a change of keys, and so on. This subject is well covered in the patent art, particularly in Class 70, sub class 383 in the U.S. Patent Office.
There have been many attempts to design and manufacture locks where the keys can be readily changed. In most of these, particularly designed for safe deposit boxes in banks, two keys are customarily employed. One key is controlled by the bank, and the other is held by the 37 renter." Both keys must be used to open the lock. When the renter terminates his period of rent, or loses a key, or for whatever reason either one or both of the keys have to be changed, the lock is so arranged that after opening the lock with both keys, the renter's key can be easily changed. Or by operating a third key, or by a special tool inserted from the back of the lock, both keys can be changed.
Samples of such locks can be seen in U.S. Pat. Nos. 1,136,067, 4,462,230, 1,693,731, 3,983,728, 3,837,196 and 4,561,269.
There have also been designs where the key can be changed by simply opening the lock with one key and closing it with another. The second key is then necessary to open the lock. An example of this design is in U.S. Pat. No. 4,712,399. A much more complicated mechanism is described in U.S. Pat. No. 3,774,424.
In most of the art cited, the change in mechanism is accomplished by the use of two-part tumblers. The two parts are normally held together by one or two sets of teeth that lock the two parts together. When it is desired to change the key, the two-part tumblers are separated and the parts are shifted relative to each other. Then the parts are recombined and the "new" tumblers are appropriate to the new key.
This is done, for example in U.S. Pat. Nos. 4,712,399 and 3,774,424.
In other designs, lever tumblers are employed where a pivot point is changed for a change in key. For examples see U.S. Pat. Nos. 1,136,067, 4,561,269 and 4,072,032.
All the locks cited above can be criticized for being complex, requiring large cases or very small parts, and in many examples being obviously unreliable due to small sizes of interlocking teeth or single point pivots .